The green line (557.7 nm) of atomic oxygen and the Herzberg bands of molecular oxygen (measured between 250 and 280 nm) as observed from the Ogo 4 airglow photometer from August 1967 through January 1968 are discussed in terms of their spatial and temporal distributions and their relation to the atomic oxygen content in the lower thermosphere. Daily maps of the distribution of emissions show considerable structure (cells, patches, and bands) with appreciable changes from day to day. When data are averaged over periods of several days in length, the resulting patterns have only occasional tendencies to follow geomagnetic parallels. The seasonal variation is characterized by maxima in both the northern and the southern hemispheres in October, the northern hemisphere having substantially higher emission rates. These maxima tend to move toward the poles, leaving very low values of emission at low latitudes in December and January. Noting the similarity of the atomic oxygen profiles in the lower thermosphere to the profile of a Chapman distribution, formulae are derived relating the vertical column emission rates of the green line and the Herzberg bands to the atomic oxygen peak density. Global averages for the time period for these data (August 1967 to January 1968), when expressed in terms of maximum atomic oxygen densities near 100 km, have a range of 2.0 X 10 • cm -3 to 2.7 X 10 • cm -3. Their variation closely follows the phase of the semiannual variation in total density observed at higher altitudes from the analysis of satellite drag data.

Ogo 4 observations of the O I (6300‐A) emissions have revealed a global pattern hitherto undetected from the ground‐based observations. It is seen that the postsunset emission of O I (6300 A) in October 1967 is very asymmetrical with respect to the geomagnetic equator in certain longitude regions and shows poor correlation with the electron density measured simultaneously from the same spacecraft. This asymmetry is less marked in the UV airglow, O I (1356 A), which appears to vary as the square of the maximum electron density in the F region. The horizon scan data of the 6300‐A airglow reveal that the latitudinal asymmetry is associated with asymmetry in the height of the O I (6300‐A) emission and hence with the altitude of the F2 peak. From the correlative studies of the airglow and the ionospheric measurements the mechanisms for the UV and the 6300‐A emissions are discussed in terms of the processes involving radiative and dissociative recombination. Theoretical expressions are developed relating the airglow data to the ionospheric parameters, and it is shown that the agreement between observed and calculated emission rates is well within the uncertainty of the measurements.

The atomic oxygen emission line at 6300 A as measured in the nadir direction by a photometer on the polar-orbiting satellite Ogo 4 has been plotted between 40øN and 40øS latitude on a series of maps for the moon-free periods between August 30, 1967, and January 10, 1968. Readily apparent are the longitudinal and local time variations that occur during the northern fall-winter season. The northern tropical arc is more widespread; the southern arc is not present at all longitudes. The arcs in early evening are strong and distinct, separated by very low emission rates at the magnetic equator. The arcs lie generally along magnetic parallels, move toward the magnetic equator as the night progresses, and, in the early morning hours, decrease in emission rate and degenerate into patches. Regions of enhanced emissions, corresponding to a sunlit atmosphere in the conjugate area, are found both in the evening and in the morning. The paper describes the conditions under which the observations were rnade as well as presenting four airglow maps selected to show the local time variations. The observations of the 6300-A emission line of atomic oxygen O(•D -) sp) on a worldwide basis from satellite-borne photometers offer a unique method of probing the atmosphere at altitudes near 300 km. At night at low latitudes the O(•D) is primarily the result of a two step process: 0"' + O• '--' O• + + 0 followed by o,.++ e -, o + O('D) The O(•D), which has a radiative lifetime of 110 see, will at altitudes below about 250 km begin to lose its energy to the neutral molecules by collisional deexcitation. Hence the õ300-A emission is a reflection of the vertical distribu-tion of both the neutral and ionized components of the atmosphere. The use of observations of 6300 A has been limited by the few low-latitude airglow observatories in operation in any given year and by the fact that many of the movements of interest are on a scale too large to be effectively studied from a single station. Satellite observations can be used for such features and give a geographic coverage on a scale hitherto unattainable. This paper presents a selection of Ogo 4 (orbiting geophysical observatory) observations of the 6300-A airglow and describes instrumental and operational limitations that govern their content. The various low-latitude phenomena evident in the Ogo 4 data are discussed and related to the detailed and long-term data available at specific locations. The emission rate of 6300 A as a function of latitude and longitude at various local times 5658 REED ET AL.: Low-LATITUDE 6300-A NIGHTGLOW 5659 is presented in a series of maps. The observations were made by the earth-looking airglow photometer on Ogo 4 from August 1967 through January 1968. Besides giving an overall view of low-latitude airglow and its large-scale phenomena, the maps also serve as an index to available Ogo 4 airglow data. Detailed data, including that for other wavelengths, are available in limited amounts from one of the authors (E. I. Reed). Reduced data will be placed in the Natio...

The possibility of using airglow techniques for estimating the F layer height and electron density is discussed by deriving a simple theoretical relationship between the height of the F2 peak and the column emission rates of the O I 6300‐Å and O I 1356‐Å lines. The feasibility of this method is demonstrated by presenting numerical calculations of the F2 peak heights and electron densities from the simultaneous measurement of O I 1356 Å and O I 6300 Å obtained from Ogo 4. The heights of the F2 peak estimated from this method are in good agreement with the values estimated from top side and bottom side ionosondes and with ion densities obtained from the retarding potential analyzer on the same spacecraft. The monitoring of these airglow emissions from a satellite therefore offers the possibility of mapping the height and density of the F2 peak over an extended area, a possibility that has been difficult to realize from top side sounders. Since the height of the F2 peak is a very sensitive indicator of the ionospheric motion, such methods can be used in the study of the global wind system.

The ozone densities between 40 and 68 km over Wallops Island, Virginia, were measured by observation of the night airglow in the 2400‐ to 3000‐A region by use of data from rocket‐borne photometers. Above 68 km, the ozone densities were too low to be detected by this instrumentation, that is, less than 5 × 1010 molecules/cm³. The expected nighttime enhancement of ozone was noted as an increase by a factor of 10 over daytime values at about 63 km. The airglow emission was found to originate above 87 km with a maximum emission rate at 93 km.